Ec. Hansen et al., DIFFERENTIATION OF LATE ARCHEAN CRUST IN THE EASTERN DHARWAR CRATON, KRISHNAGIRI-SALEM AREA, SOUTH-INDIA, The Journal of geology, 103(6), 1995, pp. 629-651
The Late Archean crust south of Krishnagiri, Tamil Nadu, consists of t
onalitic-trondhjemitic-granodioritic (TTG) gneisses with mafic and sed
imentary enclaves, formed between 2.7 and 2.5 Ga and metamorphosed at
amphibolite facies in the north to granulite facies in the south close
to 2.5 Ga. Migmatization occurred at all grades, and numerous small g
ranite bodies were emplaced near the amphibolite-to-granulite facies h
orizon. This nearly syn-accretion metamorphism affected the entire cru
st and left a chemically differentiated section later exposed by uplif
t and erosion. Detailed chemical and petrographic study of > 60 sample
s across a 90 km traverse provides evidence for an essentially unbroke
n crustal cross-section: Paleopressures range from 4 kbar (correspondi
ng to 12-14 km paleodepth) in the north to 8 kbar (corresponding to 24
-28 km paleodepth) to the south. Corresponding paleotemperatures vary
between 650 degrees and 800 degrees C across the section. Mineralogic
grade monitors, particularly increasing TiO2 content of biotite, vary
continuously southward and dominantly reflect progressive decrease of
H2O activity. The southward succession of index minerals in quartzfeld
spathic rocks, hornblende --> clinopyroxene --> orthopyroxene --> garn
et, is a consequence of continuously increasing dryness, temperature,
and pressure. LIL elements Rb, Ba, K, and Th are continuously depleted
from north to south. This pattern is a consequence of arrested upward
movement of LIL elements and volatiles in granitic magmas and/or low-
P-H2O fluids during Late Archean metamorphism. There are many indicati
ons that a metamorphic fluid of low H2O activity was an important agen
t in the crustal differentiation. (1) Thin lenses of granite and grano
diorite are severely depleted in Rb in the biotite-rich chamockite zon
e. (2) Abundant synmetamorphic quartz veins containing orthopyroxene c
annot be magmatic segregates but must have been deposited by fluids of
low P-H2O. Because of extreme insolubility of silica in pure CO2 flui
ds, concentrated chloride/carbonate brines (essentially supercritical
salt magmas) should be considered. An immiscible nearly pure CO2 phase
, evidenced by fluid inclusions, is inadequate to account for the geoc
hemical differentiation. (3) Pervasive networks of high-Ba potassium f
eldspar microveins in all of the Rb-depleted Shevaroy Hills charnockit
es are texturally in equilibrium with orthopyroxene and garnet and rep
resent the deposits of migrating grain-boundary fluids exchanging alka
lis with plagioclase. (4) Hematite-rich ilmenite and iron-poor orthopy
roxene in these charnockites strongly suggest interaction with volatil
e oxidants including H2O, CO2, and SO2. The consistent mineralogical a
nd geochemical profiles across the Krishnagiri-Salem traverse record t
he joint action of upward-migrating magmas and fluids in Archean crust
al differentiation. Voluminous deep-crustal infusion of volatile and a
lkali-rich basalts, liberated by remelting of enriched lithospheric ma
ntle shortly after primary crustal accretion is hypothesized. We infer
that processes of crustal evolution in the Late Archean may not have
been much different from those of post-archean times.